IMPACTS is a system for automatically monitoring traffic conditions over a wide area using computer image processing techniques. The system was invented by Dr Neil Hoose at University College London (UCL) and is being commercially developed by Wootton Jeffreys Consultants in the UK.This paper presents the results of a field trial was carried out on the M25 London Orbital Motorway in the UK using a live camera.The IMPACTS system is outlined briefly and the trial aims and methods are described. Results of the trial are presented showing the detection performance of the system under a range of lighting, weather and traffic conditions.The performance of the system is considered in practical terms in the context of its potential use for automatically switching warning signs.Finally a new measure of traffic conditions is presented and example data sets obtained during the trial are described.